The present invention relates to a print head for a dot matrix printer of an armature attracting type in which an electromagnet attracts an armature to move a print needle for printing operation.
U.S. Pat. No. 4,647,236 discloses a print head of an armature attractive type which comprises a plurality of print needles slidably mounted in a head body, a plurality of armatures, and a plurality of electromagnets. Each electromagnet has a core formed on a yoke which is secured to the head body and a magnetizing coil. The attracting surface of the core is located lower than the end of the bobbin of the coil to form a recess in the bobbin. The armature has a plunger inserted into the recess of the magnet. A base end of the armature is abutted on a fulcrum surface of the yoke by an armature spring. Thus, the armature is attracted by the magnet to be pivoted about the fulcrum surface.
The armature is slidably mounted in a guide groove formed in an armature based so as not to deviate during printing operation.
In another prior art, the armature is slidably mounted in a guide groove formed in the yoke secured to the head body.
However, it may occur that the armature deflects from the axis of the print needle, which is caused, for example, by error in position between the armature base or the yoke and the armature. If the armature deflects from the center of the print needle, the armature may engage two print needles, causing double printings. Since the contact area between the armature and the head of the needle reduces, contact pressure per unit area increases. As a result, the wear of the needle increases, causing the life of the needle to shorten.
In the print head of U.S. Pat. No. 5,009,529, a needle is guided by a guide bush, and an armature is guided by cams formed on a shell. Namely, the armature is guided by a member different from a guide member of the needle. Therefore, the position of the armature is liable to be deflected from the needle.
On the other hand, it is difficult to assemble the print head in which the guide groove is provided in the armature base, as described hereinafter. The armatures are mounted on the armature base. Needles and the yoke having the core on which the coils are attached are mounted on the head body. Both the armature base and head body are opposed and coupled with each other. At that time, either of the armature base and the head body must be inverted. If the armature base is inverted, armatures may fall. If the head body is inverted, needles can fall. Therefore, the print head is manually assembled by a skilled person, because it is difficult to automate the assembling of the print head.